mod_lisp
isn't like that at all. It doesn't embed a Lisp system in Apache in
any way. So what does it do?

mod_lisp is more
like FastCGI. It's a small C
module for Apache that handles a request by converting it to a
simple text format, sending it over a socket to a Lisp system, and
sending the Lisp system's response to the client. The socket is
established on demand and can be kept alive for multiple
requests.

If all you have is mod_lisp, you still need software to process the
mod_lisp protocol and send a response. mod_lisp doesn't come bundled
with any Lisp code to do that. There are several Lisp packages that
make it easy, though. I use an old
variation
of Hunchentoot. Bill
Clementson surveyed
the Lisp web options a few years ago and mentions a few other
options.

mod_lisp is not even really Lisp-specific; if you can multiplex
network connections and parse simple text, you can write something
in any language that works with mod_lisp. It's pretty likely that your
language already has an Apache module or an interface to FastCGI,
though, unless it's even more obscure than Lisp.